In recent years, in order to flexibly operate an optical communication network, an optical transmission system that includes an optical add/drop multiplexer (OADM) disposed on a transmission path that couples a plurality of terminal stations that transmit and receive wavelength division multiplexed (WDM) light has been used. For example, in submarine optical WDM transmission systems, an OADM system including a branching unit (BU) located on the main transmission path is used. A submarine optical WDM transmission system includes a main transmission path that bi-directionally couples between two trunk line stations. The BU branches a signal light having a certain wavelength transmitted from a WDM light propagating in the main transmission path and transmits the signal light. In addition, the BU adds to the WDM light a signal light having a certain wavelength and being transmitted from the branch station. In particular, such a branching unit used in the OADM system is referred to as an “OADM-BU”.
In submarine optical WDM transmission systems including the OADM-BU described above, a state in which part of a signal light input to the OADM-BU disappears may occur due to a faulty submarine optical cable, malfunction of an optical repeater that amplifies a WDM light on a transmission path and relays the WDM light, or interruption of the supply of power to the optical repeater (hereinafter referred to as an “input signal loss state”). In addition, when a submarine optical cable or an optical repeater is repaired, the supply of power to the optical repeater in the section to be repaired is stopped using the OADM-BU. Thereafter, a repair operation is performed using a cable repair ship. Accordingly, the OADM-BU enters an input signal loss state during a period for which the repair operation is being performed.
As described above, when the OADM-BU enters an input signal loss state, a signal light transmitted in a section that is not related to the faulty section may be affected and, therefore, the quality of transmission in the section may be degraded. The degradation of transmission quality is caused because the WDM light output from the OADM-BU in an input signal loss state does not include the wavelength of a signal light in an input signal loss state, and the number of wavelengths and the total power of the WDM light are reduced from those before a failure occurs. This situation is described in more detail below with reference to a specific example.
FIG. 1 is a block diagram of the configuration of a submarine optical WDM transmission system including a widely used OADM-BU. The submarine optical WDM transmission system includes a submarine optical WDM terminal station apparatus SLTE-1A disposed in a trunk line station 1A, a submarine optical WDM terminal station apparatus SLTE-1B disposed in a trunk line station 1B, and a branch station 2A. The submarine optical WDM transmission system further includes a plurality of optical repeaters R and an OADM-BU 10 on a main transmission path L1 between the submarine optical WDM terminal station apparatuses SLTE-1A and SLTE-1B. Furthermore, the submarine optical WDM transmission system includes a plurality of optical repeaters R on a branch transmission path L2 between an input/output port of the OADM-BU 10 on the branch side and a pair consisting of submarine optical WDM terminal station apparatuses SLTE-2Aa and SLTE-2Ab in the branching station 2A.
Each of the optical repeaters R on the main transmission path L1 includes two optical amplifiers that amplify a WDM light transmitted bi-directionally. Each of the optical repeaters R on the branch transmission path L2 includes two optical amplifiers that amplify a signal light bi-directionally transmitted between the trunk line station 1A and the branch station 2A via an OADM-BU 10, and includes two optical amplifiers that amplify a signal light bi-directionally transmitted between the trunk line station 1B and the branch station 2A via an OADM-BU 10.
The OADM-BU 10 includes an optical circuit 11A. The optical circuit 11A branches a WDM light transmitted from the trunk line station 1A to the trunk line station 1B to a signal light having a specific wavelength and transmits the signal light to the branch station 2A. In addition, the optical circuit 11A adds to the main transmission path L1 a signal light having a specific wavelength transmitted from the branch station 2A to the trunk line station 1B. The OADM-BU 10 further includes an optical circuit 11B. The optical circuit 11B branches a WDM light transmitted from the trunk line station 1B to the trunk line station 1A to a signal light having a specific wavelength, and transmits the signal light to the branch station 2A. In addition, the optical circuit 11A adds to the main transmission path L1 a signal light having a specific wavelength transmitted from the branch station 2A to the trunk line station 1A.
In this example, Y denotes the number of wavelengths of a signal light transmitted from the trunk line station 1A to the trunk line station 1B. In addition, the number of wavelengths of the signal light transmitted from the trunk line station 1B to the trunk line station 1A is the same number Y. Za denotes the number of wavelengths of a signal light transmitted between the trunk line station 1A and the branch station 2A, and Zb denotes the number of wavelengths of a signal light transmitted between the trunk line station 1B and the branch station 2A. Hereinafter, for simplicity, the following setting is used: Za=Zb=Z. In such setting of the numbers of wavelengths, X=Y+Z, where X represents a total number of wavelengths of a WDM light amplified by each of the optical amplifiers included in each of the optical repeaters R on the main transmission path L1.
For example, suppose that in a submarine optical WDM transmission system as described above, a failure occurs on the branch transmission path L2 that is coupled to the input/output port of the OADM-BU 10 on the branch side, as shown in FIG. 2. At that time, a signal light transmitted from the branch station 2A to each of the trunk line stations 1A and 1B is not provided to an input port of the OADM-BU 10 on the branch side. Thus, the OADM-BU 10 enters an input signal loss state. Accordingly, the total number of wavelengths of a WDM light output from the OADM-BU 10 to each of the main transmission path L1 on the side of the trunk line station 1A and the main transmission path L1 on the side of the trunk line station 1B is reduced from X (=Y+Z) before the input signal is lost, to Y after the input signal is lost. Therefore, the total power of the WDM light is reduced to Y/X=Y/(Y+Z) of the original power after the input signal is lost.
The WDM light having reduced total power propagates in the main transmission path L1 on the side of the trunk line station 1A and is input to an optical amplifier R11A of the optical repeater R next to the OADM-BU 10. In addition, the WDM light having reduced total power propagates in the main transmission path L1 on the side of the trunk line station 1B and is input to an optical amplifier R11B of the optical repeater R next to the OADM-BU 10. In general, the optical amplifying operation performed by each of the optical amplifiers R11A and R11B is controlled so that the total power of the output WDM light is constant, regardless of the total power of the input WDM light. Accordingly, under the condition that the OADM-BU 10 is in an input signal loss state due to the occurrence of a failure of the branch transmission path L2, the power of a signal light for each of the wavelengths of the WDM light output from each of the optical amplifiers R11A and R11B is increased by 10·log(X/Y) [dB] as compared with the power of a signal light for each of the wavelengths before the input signal loss state occurs.
As specific examples, the following two cases are discussed:
Case (1): X=80, Y=60, and Z=20, and
Case (2): X=80, Y=20, and Z=60.
In case (1), the power of the signal light for each of the wavelengths of the WDM light output from each of the optical amplifiers R11A and R11B is increased by 10·log(80/60)=about 1.2 [dB] after the input signal loss state has occurred.
In contrast, in case (2), the power is increased by 10·log(80/20)=about 6.0 [dB].
If the power of the signal light for each of the wavelengths output from each of the optical amplifiers R11A and R11B to the main transmission path L1 exceeds an input limiting value determined by the type of optical fiber used for the main transmission path L1, waveform distortion of the signal light occurs due to an increase in a non-linear effect occurring during transmission in the optical fiber. Accordingly, depending on the setting of the number of wavelengths, the quality of transmission of the signal light on the main transmission path L1 between the trunk line stations 1A and 1B that is not related to a failure occurring on the branch transmission path L2 may be degraded.
Furthermore, for example, if, as shown in FIG. 3, a failure occurs in the main transmission path L1 on the side of the trunk line station 1A coupled to the OADM-BU 10, the WDM light transmitted from the trunk line station 1A to the trunk line station 1B is not input to the OADM-BU 10. Thus, the OADM-BU 10 enters an input signal loss state. As a result, the total number of wavelengths of the WDM light output from the OADM-BU 10 to the main transmission path L1 on the side of the trunk line station 1B is reduced from X (=Y+Z) to Z after the input signal loss has occurred and, therefore, the total power of the WDM light is reduced to Z/X=Z/(Y+Z) of the original power after the input signal loss has occurred.
Accordingly, as in the case shown in FIG. 2, the power of the signal light for each of the wavelengths of the WDM light output from the optical amplifier R11B of the optical repeater R next to the OADM-BU 10 on the side of the trunk line station 1B is increased by 10·log(X/Z) [dB] as compared with the power of a signal light for each of the wavelengths before the input signal loss state occurs.
More specifically, in the setting of the number of wavelengths as in case (1), the power of the signal light for each of the wavelengths is increased by 10·log(80/20)=about 6.0 [dB] after the input signal loss state has occurred.
In contrast, in the setting of the number of wavelengths as in case (2), the power is increased by 10·log(80/60)=about 1.2 [dB].
If the power of the signal light for each of the wavelengths output from the optical amplifier R11B to the main transmission path L1 exceeds the input limiting value of the main transmission path L1, waveform distortion of the signal light occurs due to an increase in a non-linear effect. Accordingly, depending on the setting of the number of wavelengths, the quality of transmission of the signal light transmitted from the branch station 2A and received by the trunk line station 1B that is not related to a failure occurring on the branch transmission path L1 on the side of the trunk line station 1A may be degraded.
Although not shown, if a failure occurs on the main transmission path L1 coupled to the OADM-BU 10 on the side of the trunk line station 1B, the power of the signal light for each of the wavelengths of the WDM light output from the optical amplifier R11A of the optical repeater R next to the OADM-BU 10 on the side of the trunk line station 1A is increased by 10·log(X/Z) [dB] as compared with the power of a signal light for each of the wavelengths before the input signal loss state occurs. Accordingly, depending on the setting of the number of wavelengths, the quality of transmission of the signal light transmitted from the branch station 2A and received by the trunk line station 1A that is not directly related to a failure occurring on the branch transmission path L1 on the side of the trunk line station 1B may be degraded.
In order to prevent or reduce the above-described degradation of the transmission quality caused by an input signal loss state of an OADM-BU, the following techniques have been developed.
For example, Japanese Laid-Open Patent Publication No. 2000-312046 discloses a technique for maintaining the output of an optical repeater in a desirable state by combining a reference light having a wavelength different from that of a main signal light with a WDM light, transmitting the combined light, and controlling the gain of an optical amplifier in the optical repeater on the basis of the intensity information of the reference light.
In addition, for example, Japanese Laid-Open Patent Publication No. 2006-33412 discloses a technique for, when detecting that an input signal input to an optical repeater is lost due to the occurrence of a failure in the transmission path, stopping the transmission of a signal light by the upstream terminal station apparatus, making the intensity of the amplified spontaneous emission (ASE) light of an optical amplifier in the terminal station apparatus equal to the intensity of the signal light of one wavelength, outputting the ASE light to the transmission path, and setting the gain of an optical repeater disposed upstream of a point at which the failure has occurred on the basis of the intensity of the ASE light.